Post n. 3 update (first part)
But when did the 'primordial soup' (or prebiotic soup) theory begin?
Around 1870,
in a letter to a friend, Darwin wrote: "If (and this is a big if) we could
imagine that in a small hot pool, rich in ammonia, phosphoric salts, light,
heat, electricity, etc., a protein compound was chemically formed, ready to go
through even more complex changes [...]". But Darwin's official position
was firm and clear: in the present state of knowledge it is not possible (ultra vires) to formulate a theory of
the origin of life.
In 1924 A. I. Oparin, who at that time held the chair of Plant Biochemistry at Moscow University, translated this idea into a kind of scientific theory and published it in a book "Origine della vita" Ed.Bor. 1977: According to Oparin, on our planet carbon was bound to metals in the form of carbides. These, coming into contact with water vapour, reacted to form hydrocarbons and through subsequent reactions many other organic compounds.
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When the
temperature at the earth's surface dropped below 100°C, water began to condense
and all these compounds, contained in the atmosphere, were swept into a
'primitive boiling ocean' where they began to react to form larger and larger
molecules. The subsequent aggregation of these macromolecules would give rise
to gel particles, 'coacervates'. The organic coacervates would have absorbed
and assimilated substances from the environment and then, as they divided,
would have given rise to 'primitive organisms', some of which were capable of
metabolising. The evolutionary process and natural selection would eventually
give rise to all living organisms.
According to
Mario Ageno (Biosifica 3, 1984), a pupil of Enrico Fermi and a co-worker of
Edoardo Amaldi, an attentive and profound scholar of biophysics: "The
fundamental idea is certainly very brilliant and has lost none of its interest
even today. However, this must not make us forget that such a
"theory" passes over in silence all the great problems, all the great
challenges that the idea of an origin of life from inorganic matter by natural
causes poses to our minds".
But even
Haldane, while adding a brilliant idea, goes no further.
In 1929 J. B.
S. Haldane, without knowing Oparin's ideas, published a short article on the
origin of life. According to Haldane, the primitive atmosphere did not contain
oxygen, but probably H2 (hydrogen), H2O (water), CO2 (carbon dioxide), and
presumably also CH4 (methane) NH3 (ammonia). More complex molecules would have
been formed in the atmosphere by solar radiation. These organic compounds,
carried away by the rain, would have accumulated in the primitive ocean where
they would have reacted to form complex molecules, giving rise to a 'dilute
warm soup' where the first organisms would have originated.
The dilute
warm soup was immediately translated into 'Primordial Broth'; the metaphor was
born and the theory began.
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miller2.gif Schema dell'esp.di Miller 1 |
Around 1950,
an operational research programme began with H. Urey and S. Miller. In
particular, using a gas mixture of H2 (hydrogen), H2O (water), CH4 (methane)
NH3 (ammonia) similar to the atmospheres of Jupiter, Saturn and Uranus, with
the addition of energy (electrical discharges), Miller succeeded in producing amino acids, which are components of proteins, and many other organic substances.
Haldane's
theory of a primitive atmosphere devoid of O2 and the formation in the
atmosphere of substances fundamental to the origin of life and their collection
in a 'primordial soup' where life would have originated, thus seemed to be
confirmed.
It was at
this time that prebiological chemistry was born, with the aim of identifying,
in addition to the amino acids already discovered, the formation of molecules
fundamental to the origin of life and their synthesis in an environment similar
to that of the earth at the time of the appearance of life. In the years that
followed, a number of tests were carried out that confirmed the results of
Miller's experiment. In addition, a number of researchers carried out
'Miller-like' experiments, varying both the composition of the gas mixture and
the energy sources. All of this work confirmed that in prebiotic times, a large
number of organic substances could be synthesised on our planet, often
including amino acids.
At the same
time, the same substances, particularly amino acids, were found in meteorites (carbonaceous chondrite) dating back to the time of
the formation of our solar system. The discovery of amino acids in Miller's
experiments and their presence in meteorites demonstrated, according to
scientists, the ease with which these compounds could be synthesised. The
discoveries aroused great enthusiasm among scientists and it seemed that the
mystery of life would soon be unravelled.
But the major problems, passed over in silence by Oparin and Haldane in
their theory of the prebiotic broth, proved insurmountable not only, as we
shall see, from a chemical-physical point of view, but also from the point of
view of contemporaneity and localisation. In fact, it is unthinkable that the
processes we are going to list could have taken place at different times,
because the substances fundamental to the origin of life would have decomposed
in the meantime. Nor is it conceivable that they could have taken place in
different places because in that case the fundamental substances would never
have met.
But what are
the big problems that the prebiotic broth theory passes over in silence?
For each of
these problems, a few comments from authoritative scientists and, if any, the
solutions proposed by the supporters of the primordial broth theory.
1) The
molecules of amino acids, the components of proteins, exist in two forms,
Destro and Levo, and one is the mirror image of the other. If you prepare amino
acids in the laboratory, for example Alanine, what you get is 50% Alanine
Destro and 50% Alanine Levo. These two molecular forms have the same
chemical/physical properties and always travel together; their natural
separation in a prebiotic broth or in the laboratory is not possible unless an
asymmetric substance is present; but this would shift the problem to the origin
of the new asymmetric substance. The amino acids discovered by Miller in his now
famous experiment were also half right-handed and half left-handed, as were the
amino acids found in meteorites. So in the prebiotic world the amino acids must
have been half L and half D.
Ala
L Ala
D
The issue is that, the proteins of all living
organisms are made up of Levo amino acids.
Ala L
How did their separation come about and what happened to the Destro?
According to Dickerson (The Sciences; Gl albori della
vita, 1984):"[...] it may be that, at some time, there was primitive life,
or precursors to it, based on both D and L amino acids with a 50% probability
and that, in the end, the L amino acids prevailed over the others".
Most scientists consider this last solution
implausible. It is difficult enough to imagine the origin of a primitive life,
to imagine two, one Destro and the other Levo, is really hard.
R. A. Hegstrom and D. K. Kondepudi, addressed the
problem of asymmetry in an article in Le Scienze "La chiralità dell’universo" 1990. As the authors
illustrate, chemical compounds originate through electromagnetic interactions
of the atoms of which they are composed. During these processes parity is said
to be conserved, i.e. if a compound is formed its mirror image has the same probability
of forming.
The particles making up the atom, protons, neutrons
and electrons, are held together by various forces. Two of these forces, the
weak nuclear force and the electroweak force, do not maintain parity.
In the earth's crust of our planet there are elements
whose atoms decay and emit radiation (radioactive decay). During radioactive
decay, high-velocity electrons, β-rays (beta),
are also emitted. Without going into too
much detail, the weak nuclear force is responsible for this decay, and since it
does not maintain parity, more left-handed electrons are emitted than
right-handed ones. When β-rays strike chiral molecules they decompose them, but
being mostly left-handed they preferentially destroy a shape leaving an excess
of its mirror image. It was therefore thought that the weak nuclear force was
responsible for the asymmetry of life. It was found that the relative
difference in decomposition rates is of the order of one part in 109,
or one part in 1 000 000 000 (one billion).
The second force, the electroweak force, contributes
to the formation of compounds.
Since even this force does not preserve parity, it has
been calculated that during their formation, in prebiotic times, Levo amino
acids must have been more abundant than dextro amino acids on the order of one
in 1017, we are spared writing a 1 followed by 17 zeros.
Although these contributions are very small to
determine molecular asymmetry, Kondepudi and a collaborator, Nelson, attempted
to demonstrate theoretically that amplification processes can exist under
particular conditions. He imagines a tank in which Dextrous and Left are
competing and writes: 'And the tank should be large enough and sufficiently
well mixed (the stirring should be about 10 square kilometres in area and several
metres deep) to eliminate to a great extent the resulting effect of random
fluctuations. If all these conditions were met, the weak nuclear force should
be able to strongly influence the symmetry-breaking process over a period of
50000 to 100000 years". Kondepudi and Hegstrom conclude: "We have set
out numerous models to demonstrate how chiral asymmetry may have developed in
biomolecules. [...]. However, none of them has been able to indicate a
particular group of prebiotic compounds with all the properties required by
these models.
Robert M. Hazen took up the problem of molecular
asymmetry in 2001 in an article in Le Scienze: 'Vita dale rocce'. As the title
indicates, Hazen turns his attention to the mineral world and prefigures a
unique model. That is, he imagines that concentration, selection and synthesis
may have taken place within small air pockets of volcanic pumice or feldspathic
rocks. For these events, the author does not consider deterministic events and
instead states: 'Chance may have produced a combination of molecules that would
eventually have deserved to be called "living"'.
Hazen then tackles the problem of molecular asymmetry
by turning his attention to calcite crystals, limestones and marbles, because
these crystals form pairs of mirror faces. As he explains, the calcite crystals
were immersed in a solution containing a 50% Destro and Levo amino acid. After
24 hours, the crystal was extracted and washed and the solution analysed. The
Levo faces of the calcite mainly selected amino acid L with an excess of 40%
and vice versa. Hazen did not question the physical causes of this phenomenon
and stated: "Strangely enough, the more terraced faces were the most
selective. This fact led us to predict that the edges of the terraces might force
the amino acids to line up in neat rows on their respective faces". Since
left and right-handed crystal faces are equal in degree, he concludes: 'It was
by pure chance that the molecule destined for success developed in a crystal
face that preferred Left-handed amino acids to their Right-handed
counterparts'.
Ultimately a unique but random pattern, i.e. a
miracle.
2) There are only 20 amino acids in our proteins but,
in Miller's experiments, about 60 different amino acids were found. How did
this choice come about and why only 20 amino acids?
The prevailing
explanation is the obvious one: there were false starts that became extinct
because they could not compete with the lines that survived.
3) Reactions between amino acids for protein synthesis
all take place with the elimination of H2O. In an aqueous environment, i.e. in
the primordial soup, this reaction is not only chemically impossible but
proteins in water tend to break down into amino acids and this break down is
accelerated by heat.
According to S. Fox, proteins would have formed near
the volcanic cones at a temperature of 200°C and only later would they have
been washed away by rain and collected in the broth, where they would have formed
microspheres that were resistant to the destructive action of water.
Alternatively, it was imagined that the primordial soup was actually a pool of
water close to the ocean and subject to continuous evaporation. The problem has
also been solved by imagining secondary reactions between amino acids with
energy-rich compounds, but these steps greatly multiply the number of
reactions. Hundreds of reactions would have been needed to obtain a polymer of
40 amino acids, and this seems scarcely credible.
Ultimately, the question is still open.
4) The primitive atmosphere certainly did not contain
O2 (oxygen) and therefore the O3 (ozone) shield was absent. Ultraviolet rays,
in greater quantities than today, reached the surface of the earth. In a
primitive ocean, they reached a depth of 10 metres. Diffusion, thermal
agitation and currents would sooner or later bring all organic substances into
this band and they would be destroyed.
bocche idrotermali.jpg univeronline.it
To solve this problem, it is imagined that the first
organisms originated anchored to the bottom of shallow lagoons not much deeper
than 10 metres. For some researchers, the problem does not exist, as life would
have originated on the ocean floor near the mouths of the hydrothermal vents.
Now, it is obvious to anyone that all the hypotheses
made to fill these gaps are in fact ad
hoc modifications, often in contradiction with each other and without any
possibility of experimental verification.
In "The roots of biology" 1986 Mario Ageno
writes: "We can therefore say that at the beginning of the 1980s research
into the origin of life entered a crisis".
But in 1983 a major discovery revitalised the
prebiotic soup theory. Cech and Altmann discover 'Ribozymes'.
Nucleic acids (DNA and RNA) and proteins (enzymes) are fundamental
macromolecules for living organisms, and there is no doubt that in a primitive
organism they could not be lacking. However, while nucleic acids contain the
genetic information for the assembly of proteins, the latter are necessary for
the assembly of nucleic acids. Nucleic acids and proteins are interdependent,
i.e. one needs the other. This is what is known in Biophysics as the 'chicken
and egg problem', who appeared first?
Thomas R. Cech and Albert Altmann discovered that
certain types of RNA (Ribonucleic Acid) are capable of behaving both as nucleic
acids and as enzymes (i.e. they are chicken and egg together) and called them
"Ribozymes". At the suggestion of Walter Gilbert, the 'RNA world' was
born at that time, i.e. it was thought that life had originated, in the primordial
soup, through the spontaneous synthesis of a self-replicating RNA molecule and
that this, as it evolved, learned to synthesise proteins.
The 'RNA world' also aroused great enthusiasm, but the major problems listed above were still passed over in silence.
RNA is a long chain of nucleotides
consisting of the phosphate group, a sugar D-Ribose and one of four nitrogenous
bases: Uracyl, Adenine, Cytosine, and Guanine. Ribose
and nitrogenous bases have never been found in
'Miller's' experiments. Some experiments, which we can call 'non-Miller's'
experiments carried out in the 1960s and 1970s have highlighted the possibility
of the formation of Ribose together with other similar sugars and the formation
of different nitrogenous bases.
1) Now, Ribose is a pentose (i.e. it contains 5 Carbon
atoms) In pentoses, three carbon atoms are asymmetrical, and so there are
three chiral centres. This means that the number of possible molecules
(stereoisomers) is 23, i.e. 8, of which four are D (Destro) and four
are L (Levo), including D Ribose. From an energy point of view in a prebiotic
phase, they all have the same probability of being synthesised, so if D-Ribose
has been formed, the others must also have been formed.
How was D Ribose separated from the other 7 pentoses?
2) In these experiments, several nitrogenous bases
were obtained.
How were the four bases useful for RNA separated from
all the others?
On the formation of these substances Christian De Duve
in "Polvere vitale" 1995 writes: "[...] chemists have had some
success in producing the five components of RNA, but with little success and
under conditions at a time very different from a prebiotic scenario and
different for each substance. If you want to combine the components in the
right way, you run into other problems, of such magnitude, that no one has ever
tried to do it in a prebiotic context.
3) In order to have a functional RNA, the bonds
between these compounds to give origin to a nucleotide are not random but must
occur at specific points of the molecules. We are not going to go into this
aspect (already discussed in H1, "The RNA world in 2020") because
upstream of all this is the fact that the formation of the nucleotide must take
place with the elimination of water molecules and that this reaction in a
primordial broth is chemically impossible.
How would nucleotides have formed in the primordial soup?
4) In the absence of O2, ribose and nitrogenous bases
would have been destroyed by ultraviolet rays.
What could have done that?
There is not a single scientist who has tried to give
an answer to these problems.
In conclusion, after about a decade and despite the
contributions of many eminent researchers, the 'RNA world' proved to be another
failure for the primordial soup theory.
And Christian De Duve in 'Polvere vitale' summarises:
"It is honest to say that no mechanism has yet been found to
satisfactorily explain prebiotic RNA synthesis, despite considerable efforts by
some of the world's best chemists. Even the staunchest defenders of the RNA
world have expressed pessimistic views on the future prospects of this line of
research'.
And after a decade in 'Alle origini della vita' 2008
Christian De Duve adds: 'Despite all those efforts, attempts to reproduce RNA
synthesis under prebiotic conditions have achieved only limited success.
Researchers have assembled short RNA-like chains by means of mineral catalysts,
mostly clays, with artificially activated nucleotides as precursors and a few
selected moulds. However, the natural precursors have proved less effective,
and their synthesis under plausible conditions has so far frustrated the
researchers' ingenuity.
As a result of these difficulties, ad hoc solutions
are being sought. Some scientists have already come up with one: before the
"RNA World" perhaps there was a "pre-RNA World", which
later gave rise to the "RNA World". Thus, instead of simplifying, the
problem is complicated, much to Occam's chagrin.
Over the years, and up to the present day, the
primordial soup has passed from a puddle to an ocean, then to a swamp to
return, with the help of volcanoes, to an ocean, then back to a hot puddle
undergoing evaporation to return again to an ocean but in the ocean floor near
hydrothermal vents and finally to reservoirs for the 'RNA world'.
This constant search for ad hoc solutions only shows
that at the beginning of the new millennium the primordial soup theory is still
in deep crisis.
John Horgan in an article (Le Scienze, Quaderni n.89)
writes: "None of these theories is credible enough to be considered as a
paradigm, but none of them has been shown to be false, and this annoys Miller,
who is known to be a rigorous experimentalist, but also a rather intransigent
person".
Horgan continues: 'This approach, Miller protests,
feeds the belief that the origin of life is of interest to a fringe of the
scientific world, as a discipline unworthy of serious research.
From this statement we can deduce that there are no
institutions that pay for serious and long-term research on the origin of life,
and what is published is only the result of marginal, or to be more precise,
'idle' research.
And it is like saying that a theory is developed when
a researcher, on the fringe of his field of research, discovers a clue that
could be linked to the problem of the origin of life. And so astrophysicists
think that life comes from space and find aliens on Earth. Geneticists take an
'RNA world' for granted, forgetting that it is only a hypothetical world.
Metabolists give priority to the origin of proteins without even speculating on
their physical origin. Of course, those who have discovered hydrothermal vents
claim that life originated on the ocean floor. And all those who are baffled by
the results of research and evolutionists prefer to speculate by dusting off
the 'magic' word: chance.
And with regard to the theories on the origin of life,
what Christian De Duve writes in 'Polvere vitale' is still valid today: 'What
we have instead is a variety of theories, influenced by scientific
specialisation, philosophical attitudes and the ideological leanings of their
authors'.
In recent years, laboratory research has been greatly
reduced, favouring research using computer models with obviously contradictory
results.
In conclusion, experimental research has shown that
life could not have originated in the prebiotic broth and therefore the
prebiotic broth never existed. However, the metaphor of the primordial broth is
so powerful that it has crystallised in the minds of scientists and, lazily,
remains the most accepted theory.
Giovanni
Occhipinti
Next article end of June.
The Prebiotic Broth (Part Two): Review of experiments
and models from the last 12 years.
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